1. Field of the Invention
The present invention relates to an image fixing method for use in a wet-type electrophotographic copying machine, more particularly to an image fixing method comprising the steps of developing a latent electrostatic image to a visible toner image by use of a developer comprising a thermal cross-linking toner and a carrier liquid, transferring the visible toner image to a transfer sheet, and fixing the transferred toner image to the transfer sheet by bringing a heat application means into direct contact with the transferred image.
2. Background of the Related Art
In a wet-type electrophotographic copying machine, there is conventionally known an image fixing method comprising the steps of developing an electrostatic latent image formed on a photoconductive member to a visible toner image with a liquid-type developer, transferring the visible toner image to a transfer sheet, and fixing the transferred image to the transfer sheet by bringing a heat application means into contact with the opposite side of the transfer sheet to the toner-image-bearing side thereof.
Such a conventional image fixing method can be explained with reference to a wet-type electrophotographic copying machine as shown in FIG. 3. In this wet-type electrophotographic copying machine, a heat-application roller 21 with a heater 23 incorporated therein and a plurality of pressure-application rollers 22 are provided. By the pressure-application rollers 22 which are in contact with the toner-image bearing side of a transfer sheet P, the transfer sheet P is brought into pressure contact with the heat application roller 21. More specifically, the heat-application roller 21 with the heater 23 incorporated therein is disposed along a paper path through which the unfixed toner image bearing transfer sheet P advances in the direction of the arrow, with its back side in close contact with the heat application roller 21. On the opposite side across the paper path, a plurality of the pressure-application rollers 22 are located in order to allow the transfer sheet P to come into close contact with the heat-application roller 21. As previously mentioned, the unfixed toner image bearing transfer sheet P is heated by the heat-application roller 21 from the back side thereof. The reason for this is to prevent the unfixed toner image formed on the transfer sheet P from corrupting or flowing under the direct application of heat, because the unfixed toner image contains a considerable amount of a solvent component therein, so that the toner particles are not so tightly bonded to each other, thus the toner images are easily corrupted.
When the transferred toner image is fixed to the transfer sheet by bringing a heat application means into contact with the back side of the transfer sheet, the thermal energy supplied by the heat application means is applied to the back of the transfer sheet P to elevate the temperature thereof, and then transmitted to the unfixed toner image thereon. Therefore, the image quality of the fixed images is considerably affected by the thickness and the surface smoothness of the transfer sheet. If the thickness and the surface smoothness of the transfer sheet are not appropriate, satisfactory fixing performance may not be obtained. Furthermore, this fixing method has its limit in application in the case of high-speed copying.
Instead of the above-mentioned back-side heat application image fixing method, an image fixing method of bringing a heat application means into direct contact with the transferred image is proposed in Japanese Laid-Open Patent Application 62-139426, which is referred to as the front-side heat application image fixing method.
The mechanism of this front-side heat application image fixing method is illustrated in FIG. 4. As shown in FIG. 4, a heat-application roller 31 having an inner heater 33 therein is brought into pressure contact with a pressure-application roller 32, with the paper path of the transfer sheet P interposed therebetween. A cam surface of a pressure-application cam 35 is in pressure contact with a shaft portion of the pressure-application roller 32 by the urging force of a spring 36. Around the heat-application roller 31, a thermistor 37 and a fuse 38 are provided to control the temperature of the heat-application roller 31. A transfer sheet separating pawl 40 is situated in contact with the surface of the heat-application roller 31. A silicone oil application felt 41 is in contact with the surface of the roller 31. The transfer sheet separating pawl 40 and the silicone oil application felt 41 serve to prevent the transfer sheet from winding around the heat-application roller 31.
However, the above-mentioned direct heating method by which the surface of the unfixed toner image bearing transfer sheet is directly heated by the heat-application means has the shortcoming that the image quality of the fixed image varies depending on the kind or type of the transfer sheet P. In particular, when transfer sheets through which a liquid developer does not permeate, such as a transparent film for the overhead projector (OHP), an intermediate original sheet and art paper, are employed, the unfixed toner image on the transfer sheet is caused to disadvantageously corrupt or flow in the course of image fixing and accordingly the fixed image becomes illegible.